How to Size a Marine Lithium Battery Bank
How to size a marine lithium bank: run a load audit, convert amp-hours to watt-hours, choose days of autonomy, and check that charging can replace what you use.
Sizing is a sequence, and the sequence only runs in one direction: measure the load, convert to energy, choose autonomy, then check that generation can replace it. Skipping to a capacity figure and working backwards is the most common way boats end up with a bank that is both too big and too slow to recharge.
How do I work out what size lithium battery I need for my boat?
Work it out in four steps, in this order. First, list every load with its watts and the hours per day it runs, and multiply to get watt-hours. Second, add the list up to get daily watt-hours, then divide by 1,000 for kWh per day. Third, multiply by the days of autonomy you want between full charges. Fourth, check that your charging sources can put that much back in the time you actually have. The battery size calculator runs the arithmetic; the measuring is yours.
How do I do a load audit on a boat?
Build a table with one row per load and four columns: watts, hours per day, watt-hours per day, and how you got the figure. Read watts from the appliance label or measure with a clamp meter on the DC feed. Be honest about hours — a fridge compressor cycles, so its duty cycle matters more than its nameplate. Do the audit twice, once for a summer day at anchor and once for a winter passage, because those are different boats. Calculating boat power usage walks through a worked table.
How do I convert amp-hours to kilowatt-hours?
Multiply amp-hours by the nominal pack voltage, then divide by 1,000. A 12.8V 200Ah LiFePO4 battery is 200 × 12.8 = 2,560Wh, or 2.56kWh. This conversion is the only way to compare banks honestly, because a 200Ah 24V bank holds roughly twice the energy of a 200Ah 12V bank while carrying the same amp-hour label. Amp-hours vs watt-hours explains why every comparison on this site is made in watt-hours.
How many days of autonomy should I design for?
Choose days of autonomy from how long you actually go between reliable charges, not from a rule of thumb. A boat on a marina berth with shore power every night needs very little reserve. A boat anchored out with solar needs enough to carry the worst realistic run of overcast days for its cruising ground and season. Sizing for a week of autonomy on a boat that charges every second day buys weight and cost you never use. Cruising yacht battery reserve works through the trade.
How do I check whether my charging can keep up with the bank?
Compare daily generation against daily consumption in watt-hours, using the worst month you cruise in, not the annual average. Add what solar produces at winter sun-hours, what the alternator returns in the engine hours you genuinely run, and what shore power contributes if you get it. If generation is below consumption, a larger bank only delays the deficit — it does not remove it. Size the charging first, then the bank. The solar sizing calculator and alternator charging calculator give both halves.
How much usable capacity does a LiFePO4 bank actually have?
Usable capacity depends on the depth of discharge the manufacturer warrants and the point at which your BMS disconnects, so read both before assuming a figure. LiFePO4 tolerates far deeper discharge than lead-acid, which is a large part of why a smaller lithium bank can replace a bigger AGM one, but "usable" is still a specification rather than a constant. Check the datasheet's cycle-life curve at the depth of discharge you intend to use daily. What is LiFePO4 covers the chemistry behind the difference.
How do I size a bank when I do not know my loads yet?
Size provisionally, then re-measure within the first season. Start from a comparable boat with a similar fridge, autopilot and electronics fit, install a battery monitor with a shunt from day one, and log real consumption for a month of normal use. Leave physical space and cable capacity for one more battery so the bank can be extended rather than replaced. A provisional bank plus a shunt beats a confident guess with no measurement.
How do I decide between one large battery and several smaller ones?
Decide on serviceability, physical access and BMS behaviour rather than on capacity, because the capacity can be reached either way. Several smaller batteries can be carried aboard by one person and let a single failed unit be isolated; one large battery has fewer connections to fail and one BMS to configure. Parallel banks need matched batteries and careful symmetrical cabling so current shares evenly. One large battery vs parallel batteries sets out both cases.
How do I size a bank for a liveaboard rather than a weekender?
Size a liveaboard from measured daily kWh across a full week of ordinary living, including cooking, hot water, laundry and work-from-boat electronics. A weekender's audit is dominated by the fridge and the lights; a liveaboard's is dominated by whichever comfort load you are unwilling to give up. Because liveaboard consumption is continuous rather than episodic, recharge rate matters as much as capacity. Best lithium battery for a liveaboard and off-grid liveaboard configurations show how the two figures interact.
How do I size a bank for a catamaran with two engines?
Treat the two engines as two charging sources with separate alternator paths, and size the house bank from the combined load audit of both hulls. Catamarans typically offer more deck area for solar and more locker volume for batteries than a monohull of the same length, which usually shifts the answer toward more solar rather than more battery. Cable runs between hulls are long, so voltage drop and conductor cost often push a catamaran toward 24V or 48V. Catamaran lithium battery system covers the layout.
How do I size a bank when I will add air conditioning later?
Size the cabling, busbars and physical space now for the system you intend to end with, and add battery capacity when the air conditioning goes in. Air conditioning is usually the largest single load a cruising boat adds, so it often changes the system voltage decision as well as the bank size. Retrofitting cable and reworking the distribution is far more expensive than buying the right conductor once. Future-proofing a marine lithium installation and air conditioning on lithium cover the sequencing.
How do I know if my bank is oversized?
A bank is oversized if it routinely sits above a high state of charge and never cycles through the range you paid for. Watch the shunt over a month: if the bank never drops below a high state of charge even in your worst weather, you are carrying capacity, weight and cost you do not use. Oversizing also slows charging in practice, because a fixed alternator or array output has more capacity to fill. The fix is usually more load or a smaller next bank, not more charging.
How do I compare two quoted banks that use different voltages?
Convert both to kilowatt-hours before anything else, then compare on the same four axes: usable energy, continuous BMS current, charge acceptance, and what else each quote includes. A 48V bank quoted at 100Ah and a 12V bank quoted at 400Ah are not close in energy, but their amp-hour labels invite the comparison. Quotes also differ in what they include beyond batteries. Comparing marine lithium batteries and hidden costs of a lithium conversion cover both halves.